Such a hydraulic oil filtration apparatus is conventionally constructed, for example, as shown in FIG. 7. This apparatus comprises a hydraulic actuator A, a hydraulic pump P, an actuation control valve V, a pump-pressure oil passage 41, a connection oil passage 101, a tank oil passage 51, a return filter 100, a drain oil passage 53 and a drain filter 102. The hydraulic actuator A functions as a drive in an assembly of industrial equipment, which is mounted on a vehicle, and the actuation control valve V controls to charge and discharge the hydraulic actuator A with hydraulic oil delivered by the hydraulic pump P The pump-pressure oil passage 41 leads the hydraulic oil discharged from hydraulic pump P to the actuation control valve V, and the connection oil passage 101 is positioned between the hydraulic actuator A and the actuation control valve V, to let the hydraulic oil move therebetween for the actuation of the hydraulic actuator A. The tank oil passage 51 connects the actuation control valve V to a tank T to lead the hydraulic oil that is discharged from the hydraulic actuator A during the actuation thereof, through the connection oil passage 101 and the actuation control valve V The return filter 100 is positioned in the tank oil passage 51 to clean the oil discharged from the hydraulic actuator A. The drain oil passage 53 connects a hydraulic motor M constituting the hydraulic actuator A to the hydraulic pump P and thereby to the tank T to return oil that has leaked from the hydraulic pump P and the hydraulic motor M, to the tank T. The drain filter 102 is positioned in the drain oil passage 53 to clean the oil that has leaked from the hydraulic pump P and the hydraulic motor M.
The return filter 100 of the hydraulic oil filtration apparatus comprises a return casing 100a, a return filter element 100b and a return head 100c. The return casing 100a is a container with an upper opening and accommodates the return filter element 100b, and the return head 100c is mounted removably to cover the opening of the casing 100a. Also, the drain filter 102 comprises a drain casing 102a, a drain filter element 102b and a drain head 102c in the same manner as the return filter 100. The drain casing 102a is a container with an upper opening and accommodates the drain filter element 102b, and the drain head 102c is mounted removably to cover the drain casing 102a. 
In this hydraulic oil filtration apparatus, oil that has been discharged in the actuation of the hydraulic actuator A passes through the connection oil passage 101, the actuation control valve V and the tank oil passage 51, where the oil is cleaned by the return filter 100, and the oil returns to the tank T. On the other hand, oil that has leaked from the hydraulic motor M and the hydraulic pump P passes through the drain oil passage 53, where the oil is cleaned by the drain filter 102, and the oil returns to the tank T.
Because the amount of the oil that leaks from the hydraulic pump and the hydraulic motor is small as compared with the amount of the oil that is discharged in the actuation of the hydraulic actuator, the amount of the leakage oil that is cleaned by the drain filter and returned to the tank is also small. Here, if the leakage oil that has passed through the drain filter 102 is directed into the tank oil passage 51, which leads from the return filter 100 to the tank, then the material for constructing the oil passages can be economized. However, if the apparatus were arranged in this manner, then it would present a problem of the oil passing through the return filter 100 flowing into the drain filter 102, resulting in an impairment of the function of the drain filter 102.
Furthermore, when the filter elements of the return filter and the drain filter are to be replaced, it is necessary to remove the return casing from the return head for the replacement of the return filter element together with the return casing as well as to remove the drain casing from the drain head for the replacement of the drain filter element together with the drain casing. In a case where the return filter and the drain filter are provided independently away from each other, a worker, after replacing one of the filters, must move to the location where the other filter is installed before engaging in the replacement of the other filter. This is a problem that impairs the efficiency of the replacement work.